Climate change takes a hidden toll on California water supplies

(CN) — California's notorious droughts are a major concern for farming communities and water policymakers, but a new study emphasizes that it's not just precipitation rates taking a toll on the agricultural water balance: Crop water demand is exacting an "invisible water surcharge" that explains half of the cumulative deficits of that water supply since 1980.

The amount of water a plant needs is determined by the rate of its evapotranspiration, which is the combination of evaporation from soil and plant surfaces and water absorbed by plant roots that is released through the leaves as a vapor.

As temperatures increase due to climate change, the atmosphere can hold more moisture creating a vapor pressure difference that can draw moisture from plant and soil surfaces.

These climate-induced changes to evapotranspiration are playing an increasing role in the growing water deficit in San Joaquin Valley, according to a new study published Wednesday in the Public Library of Science Water journal.

In the past decade, the average annual climate-driven increase in crop evapotranspiration is enough to drain a major reservoir, and over a 12-year period, the study's authors estimate the equivalent volume of San Joaquin Valley’s five largest reservoirs is lost to climate-induced evapotranspiration.

The San Joaquin Valley is home to some of the state's most productive agricultural land, but the basin does not receive enough rainfall to support the region’s intensive production of primarily fruit and nut crops, as well as beans, corn, alfalfa, squash, melons and more.

The additional water required to irrigate those crops must be imported through the network of levies, reservoirs and aqueducts that convey water from Sierra Nevada watersheds in the north of the state to dryer agricultural regions in the south, or water is pumped from wells that tap the valley’s network of underground aquifers.

Those supplemental water sources have uncertain futures. A century of overreliance on groundwater pumping has seriously depleted the valuable resource, leaving wells dry and causing land to sink or settle as aquifers collapse.

The environmental damage caused by groundwater overdraft and the disproportionate use that has left some areas without water entirely has led to state regulations such as the Sustainable Groundwater Management Act passed in 2014.

As the state works to slow the impacts of groundwater pumping, climate change has led to unstable snowpacks in the Sierra Nevada with warmer spring weather leaving less snowmelt to recharge reservoirs in mid and late summer when irrigation demand is high.

In addition to the interconnected challenges of drought and water storage to supply productive agricultural regions, temperatures in California have increased by 2.88 degrees Fahrenheit since 1900 and these temperature changes have been affecting crop evapotranspiration especially in the past decade, according to study's authors.

The additional irrigation water needed to compensate for increases in evapotranspiration is placing and added burden on troubled groundwater sources and precarious imports from the Northern Sierra watershed.

Authors of the study call this additional water requirement the invisible water surcharge because it is hidden within overall water demand.

“It’s also invisible because it’s in the atmosphere and we really don’t see it,” said Dr. Joshua Viers, associate dean of research at University of California, Merced, who is the senior author of the study. “But the warmer conditions are, in effect, putting a multiplier on what we would have considered to be normal conditions and that is driving additional demand that requires additional irrigation otherwise crops may not produce as well.”

In order to isolate the invisible water surcharge from the San Joaquin Valley’s total agricultural water demand, researchers modeled growth stages of 17 major crop categories grown in the valley and used cropping information and daily surface meteorological data to derive crop evapotranspiration rates across a 40-year period.

The study period spanned from 1980 to 2023. During the first three decades, crop evapotranspiration increased as researchers expected, but after 2011, the rate dramatically changed.

“We were able to detect a regime shift,” Viers said. “Such that the entire relationship across wet and dry years is elevated by 4.4% which comes out to just less than two inches" of water loss related to crop evapotranspiration.

"Which doesn’t seem like a lot, but when that’s over 5 million acres it adds up significantly," Viers added.

California’s multi-billion dollar agricultural industry is responsible for a significant portion of the global food supply as a major producer almonds, pistachios and fruit. For the authors of the study, this underscores the importance of taking climate-induced increases in crop water demand into account in regional water management plans established under the Sustainable Ground Water Management Act.

Viers noted that the Public Policy Institute of California has estimated that about 500,000 acres or 10% of the San Joaquin Valley study area would need to come out of production to bring irrigation demand into balance.

“I think we would suggest an ‘all of the above’ approach," Viers said. This would include investment in conveyance infrastructure and technology that can optimize reservoir releases in coordination with snowpack and hydropower needs as well as technology that can allow for better measurement and accounting of water throughout the system.

Carefully managed groundwater use can be combined with strategic recharge of aquifers in wet years, and management solutions like modifying the mix of annual and perennial crops could also be of value in mitigating water losses, according to study authors.

Mediterranean climates around the world may face similar increases in crop water demand, and the authors recommend further study of agricultural regions in South Africa, Chile, Australia and the Mediterranean Basin.

“Many fruit and nut crops can only be grown in a Mediterranean climate,” the study's authors wrote. “Thus, the invisible water surcharge has important implications on the types of crops within our global food supply.”

Subscribe to Closing Arguments

Sign up for new weekly newsletter Closing Arguments to get the latest about ongoing trials, major litigation and hot cases and rulings in courthouses around the U.S. and the world.

Email *

Submit